Method of quasi-atomic layer etching of silicon nitride

ABSTRACT

A method of etching is described. The method includes providing a substrate having a first material containing silicon nitride and a second material that is different from the first material, forming a first chemical mixture by plasma-excitation of a first process gas containing H and optionally a noble gas, and exposing the first material on the substrate to the first chemical mixture. Thereafter, the method includes forming a second chemical mixture by plasma-excitation of a second process gas containing N, F, O, and optionally a noble element, and exposing the first material on the substrate to the second plasma-excited process gas to selectively etch the first material relative to the second material.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority of each of the followingapplications and describes the relationship of the earlier applications.The present application is a Continuation Application of and claims thebenefit of priority from co-pending U.S. application Ser. No.15/686,546, filed Aug. 25, 2017, and also claims the benefit of priorityfrom U.S. Provisional Application No. 62/380,567, filed Aug. 29, 2016.The entire contents of all of the foregoing applications areincorporated herein by reference.

FIELD OF INVENTION

The invention relates to a method for etching, and more particularly, aprecision etch technique for etching a thin film for electronic deviceapplications.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing asemiconductor device such as an integrated circuit and transistors andtransistor components for an integrated circuit. In the manufacture of asemiconductor device (especially on the microscopic scale), variousfabrication processes are executed such as film-forming depositions,etch mask creation, patterning, material etching and removal, and dopingtreatments, are performed repeatedly to form desired semiconductordevice elements on a substrate. Historically, with microfabrication,transistors have been created in one plane, with wiring/metallizationformed above, and have thus been characterized as two-dimensional (2D)circuits or 2D fabrication. Scaling efforts have greatly increased thenumber of transistors per unit area in 2D circuits, yet scaling effortsare running into greater challenges as scaling enters single digitnanometer semiconductor device fabrication nodes. Semiconductor devicefabricators have expressed a desire for three-dimensional (3D)semiconductor devices in which transistors are stacked on top of eachother.

As device structures densify and develop vertically, the need forprecision material etch becomes more compelling. Trade-offs betweenselectivity, profile, ARDE (aspect ratio dependent etching), anduniformity in plasma etch processes become difficult to manage. Currentapproaches to patterning and pattern transfer by balancing thesetrade-offs is not sustainable. The root cause for these trade-offs isthe inability to control ion energy, ion flux, and radical fluxindependently. However, self-limiting processes, such as atomic layeretching (ALE), offer a viable route to escape these trade-offs bydecoupling the etch process into sequential steps of surfacemodification and removal of modified surface regions, thereby allowingthe segregation of the roles of radical flux and ion flux and energy.

SUMMARY

Techniques herein pertain to device fabrication using precision etchtechniques.

A method of etching is described. The method includes providing asubstrate having a first material containing silicon nitride and asecond material that is different from the first material, forming afirst chemical mixture by plasma-excitation of a first process gascontaining H and optionally a noble gas, and exposing the first materialon the substrate to the first chemical mixture. Thereafter, the methodincludes forming a second chemical mixture by plasma-excitation of asecond process gas containing N, F, O, and optionally a noble element,and exposing the first material on the substrate to the secondplasma-excited process gas to selectively etch the first materialrelative to the second material.

Of course, the order of discussion of the different steps as describedherein has been presented for clarity sake. In general, these steps canbe performed in any suitable order. Additionally, although each of thedifferent features, techniques, configurations, etc. herein may bediscussed in different places of this disclosure, it is intended thateach of the concepts can be executed independently of each other or incombination with each other. Accordingly, the present invention can beembodied and viewed in many different ways.

Note that this summary section does not specify every embodiment and/orincrementally novel aspect of the present disclosure or claimedinvention. Instead, this summary only provides a preliminary discussionof different embodiments and corresponding points of novelty overconventional techniques. For additional details and/or possibleperspectives of the invention and embodiments, the reader is directed tothe Detailed Description section and corresponding figures of thepresent disclosure as further discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates a schematic representation of a method of etching athin film on a substrate according to an embodiment;

FIG. 2 provides a flow chart illustrating a method of etching asubstrate according to an embodiment;

FIG. 3 illustrates a result obtained using the method of etchingdepicted in FIGS. 1 and 2;

FIGS. 4 and 5 illustrate additional results obtained using the method ofetching depicted in FIGS. 1 and 2;

FIGS. 6A through 6D illustrate various exemplary fabrication sequencesto which the method of etching depicted in FIGS. 1 and 2 can be appliedaccording to several embodiments;

FIGS. 7A and 7B illustrate a schematic representation of a method ofetching a thin film on a substrate according to another embodiment;

FIG. 8 provides a flow chart illustrating a method of etching asubstrate according to yet another embodiment; and

FIGS. 9A through 9D provide schematic illustrations of plasma processingsystems for performing the method of etching according to variousembodiments.

DETAILED DESCRIPTION

Techniques herein pertain to device fabrication using precision etchtechniques. Several instances manifest in semiconductor manufacturing inboth front end of line (FEOL, e.g., transistor fabrication) through tothe back end of line (BEOL, e.g., interconnect fabrication), where oxideand nitride films (typically silicon-containing, in nature) need to beetched with a high degree of precision.

Numerous fabrication sequences in semiconductor manufacturing demandprecision etch techniques. Examples, which will be discussed later,include: (1) gate spacer etch for both 2D (two-dimensional) and 3D(three-dimensional) device structures, (2) spacer etch for sidewallimage transfer (SIT) for multi-patterning, (3) mandrel removal from apost-spacer etch SIT structure, and (4) liner etch from a raisedstructure.

As another example, fabrication of self-aligned block (SAB) structureshas become a critical step in self-aligned double patterning (SADP),self-aligned quadruple patterning (SAQP), and other variations ofself-aligned multiple patterning (SAMP). As part of an SAB flow, asilicon nitride mandrel is anisotropically etched with selectivity to anoxide spacer. Current approaches to etch silicon nitride mandrel do nothave the required selectivity, which exceeds 15 (i.e., the etch rate ofsilicon nitride is 15 times greater than the etch rate of silicon oxide)to etch the mandrel without damaging the oxide spacer.

This invention relates to development of an anisotropic process that canetch silicon nitride mandrel with extremely high selectivity (e.g., >15,or >20, or >30, or >50, or >80, and even >100) to oxide spacer, therebyenabling SAB fabrication flows.

According to several embodiments, FIGS. 1 and 2 illustrate a method ofetching a thin film. The method, depicted as flow chart 200, includesproviding a substrate having a first material 100 containing siliconnitride and a second material (not shown) that is different from thefirst material 100, forming a first chemical mixture byplasma-excitation of a first process gas containing H and optionally anoble gas in step 210, and exposing the first material on the substrateto the first chemical mixture in step 220, the combination of which isdepicted as 102 in FIG. 1. Thereafter, the method includes forming asecond chemical mixture by plasma-excitation of a second process gascontaining N, F, O, and optionally a noble element in step 230, andexposing the first material 100 on the substrate to the secondplasma-excited process gas to selectively etch the first material 100relative to the second material in step 240, the combination of which isdepicted as 104 in FIG. 1.

The first material 100, to be etched, contains, consists essentially of,or consists of silicon nitride, expressed as Si₃N₄, or more genericallySi_(x)N_(y), wherein x and y are real number greater than zero. Thesecond material (not shown) can include silicon oxide, e.g., SiO₂, orother silicon-containing material, a metal or metal-containing material,or an organic material, such as an organic planarization layer (OPL),resist, or antireflective coating (ARC).

As set forth above, the first chemical mixture is formed from the plasmaexcitation of a first process gas. The first process gas containshydrogen (H), and can include atomic hydrogen (H), molecular hydrogen(H₂), metastable hydrogen, hydrogen radical, or hydrogen ions, or anycombination of two or more thereof. In one embodiment, the first processgas includes H₂, or H₂ and Ar. In another embodiment, the first processgas consists essentially of or consists of H₂. In yet anotherembodiment, the first process gas consists essentially of or consists ofH₂ and Ar.

As also set forth above, the second chemical mixture is formed from theplasma excitation of a second process gas. The second process gascontains nitrogen (N), fluorine (F), and oxygen (O), and can optionallyinclude a noble element, such as Ar (argon). In one embodiment, thesecond process gas includes NF₃, O₂, and Ar. In another embodiment, thesecond process gas consists essentially of or consists of NF₃, O₂, andAr.

The plasma-excitation of the first process and/or the second process gascan be performed in-situ (i.e., the first and/or second chemical mixtureis formed within a gas-phase, vacuum environment in proximate contactwith the substrate), or ex-situ (i.e., the first and/or second chemicalmixture is formed within a gas-phase, vacuum environment remotelylocated relative to the substrate). FIGS. 9A through 9D provide severalplasma generating systems that may be used to facilitateplasma-excitation of a process gas. FIG. 9A illustrates a capacitivelycoupled plasma (CCP) system, wherein plasma is formed proximate asubstrate between an upper plate electrode (UEL) and a lower plateelectrode (LEL), the lower electrode also serving as an electrostaticchuck (ESC) to support and retain the substrate. Plasma is formed bycoupling radio frequency (RF) power to at least one of the electrodes.As shown in FIG. 9A, RF power is coupled to both the upper and lowerelectrodes, and the power coupling may include differing RF frequencies.Alternatively, multiple RF power sources may be coupled to the sameelectrode. Moreover, direct current (DC) power may be coupled to theupper electrode.

FIG. 9B illustrates an inductively coupled plasma (ICP) system, whereinplasma is formed proximate a substrate between an inductive element(e.g., a planar, or solenoidal/helical coil) and a lower plate electrode(LEL), the lower electrode also serving as an electrostatic chuck (ESC)to support and retain the substrate. Plasma is formed by coupling radiofrequency (RF) power to the inductive coupling element. As shown in FIG.9B, RF power is coupled to both the inductive element and lowerelectrode, and the power coupling may include differing RF frequencies.

FIG. 9C illustrates a surface wave plasma (SWP) system, wherein plasmais formed proximate a substrate between a slotted plane antenna and alower plate electrode (LEL), the lower electrode also serving as anelectrostatic chuck (ESC) to support and retain the substrate. Plasma isformed by coupling radio frequency (RF) power at microwave frequenciesthrough a waveguide and coaxial line to the slotted plane antenna. Asshown in FIG. 9C, RF power is coupled to both the slotted plane antennaand lower electrode, and the power coupling may include differing RFfrequencies.

FIG. 9D illustrates remote plasma system, wherein plasma is formed in aregion remote from a substrate and separated from the substrate by afilter arranged to impede the transport of charged particles from theremote plasma source to a processing region proximate the substrate. Thesubstrate is supported by a lower plate electrode (LEL) that also servesas an electrostatic chuck (ESC) to retain the substrate. Plasma isformed by coupling radio frequency (RF) power to a plasma generatingdevice adjacent the remotely located region. As shown in FIG. 9D, RFpower is coupled to both the plasma generating device adjacent theremote region and lower electrode, and the power coupling may includediffering RF frequencies.

The plasma processing systems of FIGS. 9A through 9D are intended to beillustrative of various techniques for implementing the steppedion/radical process described. Other embodiments are contemplatedincluding both combinations and variations of the systems described.

Turning now to FIG. 3 and Table 1, a silicon nitride film, deposited bychemical vapor deposition (CVD) (CVD SiN), is exposed to several etchingprocesses together with an adjacent silicon oxide film. In a firstexample, the two films are exposed to a hydrogen (H₂) plasma only,according to the conditions provided in Table 1. In this ion-driven,hydrogen plasma the two films are not etched and no selectivity betweenfilms is observed. In a second example, the two films are exposed toplasma composed of NF₃ and O₂. In this radical-driven plasma, eleven(11) Angstroms are etched from the silicon nitride film and only one (1)Angstrom is etched from the silicon oxide film, thus, leading to an etchselectivity of 11-to-1. In a third example, the two films aresequentially exposed to the hydrogen (H₂) plasma, and then exposed tothe plasma composed of NF₃ and O₂. In this radical and ion-drivensequential plasma, sixty one (61) Angstroms are etched from the siliconnitride film and substantially no etching is observed of the siliconoxide film, thus, leading to an etch selectivity exceeding 60-to-1.

TABLE 1 Tool: CCP, ICP, RLSA H2 plasma: 20-100 mT, HF 0 W, LF 25-100 W,500H2/50Ar, 15 C., 15-60 sec NF3—O2 plasma: 100-500 mT, HF 0-1000 W, LF15-100 W, 480NF3/160O2/1000Ar, 15 C., 5-60 Sec Dominant plasma EtchAmount [Å] Selectivity Step species CVD SiN Oxide SiN-Oxide H2 plasmaonly Ion-driven −0.6 −3 No sputtering NF3—O2 plasma Radical-driven 111 >11 only H2 + NF3—O2 61 −1 >60 plasma

The inventors surmise that hydrogen ions during the hydrogen plasma stepenrich a surface region of the silicon nitride and the silicon oxide,leading to elevated sub-surface hydrogen concentrations; see FIGS. 4 and5. As shown in FIG. 5, the hydrogen content increases in region 1(heavily modified sub-surface region) to a maximum, then decays throughmoderate concentration levels in region 2 (moderately modifiedsub-surface region), until it decays to low levels in region 3 (pristineor original material). Then, the NF₃ and O₂ plasma creates radicals thatselectively react with the hydrogenated silicon nitride and volatilizeat a rate greater than with the second material, e.g., silicon oxide ororganic material. FIG. 3 illustrates the etch amounts achieved with eachexemplary process. And, as shown in FIG. 4, the etch amount achievedduring the NF₃ and O₂ step decreases (or the etch rate decays) as theetching proceeds through the sub-surface regions from relatively highhydrogen concentration to relatively low hydrogen concentration.

In FIGS. 6A through 6D, several examples are provide of fabricationsequences in semiconductor manufacturing that demand precision etchtechniques. In each example, it is necessary to remove silicon nitridewith high selectivity to other materials, and the examples include: (1)gate spacer etch for both 2D (two-dimensional) and 3D(three-dimensional) device structures, (2) spacer etch for sidewallimage transfer (SIT) for multi-patterning, (3) mandrel removal from apost-spacer etch SIT structure, and (4) liner etch from a raisedstructure. FIG. 6A illustrates selectively removing a silicon nitride615 from the cap region of the gate structure 610. FIG. 6B illustratesselectively removing a silicon nitride 625 from a cap region and footerregion surrounding a mandrel 620 utilized in a self-alignedmulti-patterning (SAMP) scheme. FIG. 6C illustrates selectively removinga silicon nitride mandrel 635 from a post-spacer etch structure 630 toleave behind double patterned spacer structures. FIG. 6D illustratesselectively removing silicon nitride liners 645 to leave behind a raisedfeature 640.

In yet another example, the fabrication of self-aligned block (SAB)structures has become a critical step in self-aligned double patterning(SADP), self-aligned quadruple patterning (SAQP), and other variationsof self-aligned multiple patterning (SAMP). As part of an SAB flow, asilicon nitride mandrel is anisotropically etched with selectivity to anoxide spacer. Current approaches to etch silicon nitride mandrel do nothave the required selectivity, which exceeds 15 (i.e., the etch rate ofsilicon nitride is 15 times greater than the etch rate of silicon oxide)to etch the mandrel without damaging the oxide spacer.

As shown in FIG. 7A, a substrate 700 can include a patterned layer 720overlying a film stack 710, including one or more optional layers 712,714, 716 to be etched or patterned. The patterned layer 720 can definean open feature pattern overlying one or more additional layers. Thesubstrate 700 further includes device layers. The device layers caninclude any thin film or structure on the workpiece into which a patternis to be transferred, or a target material is to be removed.Furthermore, the patterned layer 720 can include a retention layer 722,and a target layer 724 to be removed.

The target layer 724 can be composed of silicon nitride. As shown inFIGS. 7A and 7B, the target layer 724 fills a trench or via 725 withinretention layer 722, the trench or via 725 has a depth (D) 727, a width(W) 726, and an aspect ratio (D/W). The aspect ratio can be greater than3, 4, or 5. For some structures, the aspect ratio can be greater than10, 15, or even 20. The width (W) 726 can be less than 50 nm, 40 nm, 30nm, or 20 nm. In some applications, the width (W) 726 is less than 10nm. The retention layer 722 can be composed of material selected fromthe group consisting of silicon oxide (SiO_(x)), silicon oxynitride(SiO_(x)N_(y)), transition metal oxide (e.g., titanium oxide (TiO_(x))),transition metal nitride (e.g., titanium nitride (TiN_(y))), andsilicon-containing organic material having a silicon content rangingfrom 15% by weight to 50% by weight silicon.

As an example, the patterned layer 720 in FIG. 7A can include a spacerlayer surrounding a mandrel layer used in multi-patterning schemes.Alternatively, for example, the patterned layer 720 in FIG. 7A caninclude a dummy silicon nitride layer filling a region to be replacedwith an advanced gate structure, such as a metal gate structure.

The substrate 700 can include a bulk silicon substrate, a single crystalsilicon (doped or un-doped) substrate, a semiconductor-on-insulator(SOI) substrate, or any other semiconductor substrate containing, forexample, Si, SiC, SiGe, SiGeC, Ge, GaAs, InAs, InP, as well as otherIII/V or II/VI compound semiconductors, or any combination thereof(Groups II, III, V, VI refer to the classical or old IUPAC notation inthe Periodic Table of Elements; according to the revised or new IUPACnotation, these Groups would refer to Groups 2, 13, 15, 16,respectively). The substrate 700 can be of any size, for example, a 200mm (millimeter) substrate, a 300 mm substrate, a 450 mm substrate, or aneven larger substrate. The device layers can include any film or devicestructure into which a pattern can be transferred.

Organic layer 721 blankets various regions of substrate 700, and exposesblock regions within which the silicon nitride mandrel is to be removedfrom high aspect ratio features. In FIG. 7B, the silicon nitride mandrel714 is selectively removed with minimal impact to the silicon oxidespacers and the organic layer 721.

FIG. 8 depicts a flow chart 800 for etching a substrate according toanother embodiment. In 810, a self-aligned block (SAB) structure isprepared. And, in 820, a mandrel is removed from an exposed region ofthe SAB structure. FIG. 8 depicts a method of selectively etching asilicon nitride mandrel from a high aspect ratio feature to leave behindsilicon oxide spacers. The aspect ratio can exceed ten (10, and the etchselectivity for removing the silicon nitride mandrel relative to othermaterials, e.g., silicon oxide and organic material, can exceed 20-to-1,or 50-to-1, or even 100-to-1.

In the claims below, any of the dependents limitations can depend fromany of the independent claims.

In the preceding description, specific details have been set forth, suchas a particular geometry of a processing system and descriptions ofvarious components and processes used therein. It should be understood,however, that techniques herein may be practiced in other embodimentsthat depart from these specific details, and that such details are forpurposes of explanation and not limitation. Embodiments disclosed hereinhave been described with reference to the accompanying drawings.Similarly, for purposes of explanation, specific numbers, materials, andconfigurations have been set forth in order to provide a thoroughunderstanding. Nevertheless, embodiments may be practiced without suchspecific details. Components having substantially the same functionalconstructions are denoted by like reference characters, and thus anyredundant descriptions may be omitted.

Various techniques have been described as multiple discrete operationsto assist in understanding the various embodiments. The order ofdescription should not be construed as to imply that these operationsare necessarily order dependent. Indeed, these operations need not beperformed in the order of presentation. Operations described may beperformed in a different order than the described embodiment. Variousadditional operations may be performed and/or described operations maybe omitted in additional embodiments.

“Substrate” or “target substrate” as used herein generically refers toan object being processed in accordance with the invention. Thesubstrate may include any material portion or structure of a device,particularly a semiconductor or other electronics device, and may, forexample, be a base substrate structure, such as a semiconductor wafer,reticle, or a layer on or overlying a base substrate structure such as athin film. Thus, substrate is not limited to any particular basestructure, underlying layer or overlying layer, patterned orun-patterned, but rather, is contemplated to include any such layer orbase structure, and any combination of layers and/or base structures.The description may reference particular types of substrates, but thisis for illustrative purposes only.

Those skilled in the art will also understand that there can be manyvariations made to the operations of the techniques explained abovewhile still achieving the same objectives of the invention. Suchvariations are intended to be covered by the scope of this disclosure.As such, the foregoing descriptions of embodiments of the invention arenot intended to be limiting. Rather, any limitations to embodiments ofthe invention are presented in the following claims.

1. A method of etching, comprising: providing a substrate in a plasmaprocessing apparatus, wherein the substrate includes a first materialcontaining silicon nitride and a second material; wherein the secondmaterial is present on a first side and a second side of the firstmaterial with the first side spaced from the second side in a lateraldirection such that the first material is between the second material onthe first and second sides; and wherein the second material comprises amaterial selected from the group consisting of: silicon oxide, siliconoxynitride, a transition metal oxide, a transition metal nitride, and asilicon-containing organic material having a silicon content of 15% to50% by weight silicon; the method further comprising: forming a firstchemical mixture in the plasma processing apparatus by plasma-excitationof a first process gas comprising hydrogen and optionally a noble gas;exposing the first material on the substrate to the first chemicalmixture in the plasma processing apparatus to hydrogenate a depth withinthe first material positioned between the second material on the firstand second sides; after the exposing the first material to the firstchemical mixture, forming a second chemical mixture in the plasmaprocessing apparatus by plasma-excitation of a second process gascontaining N and F, and optionally a noble element; and exposing thefirst material on the substrate to the second plasma-excited process gasin the plasma processing apparatus to selectively etch at least part ofthe hydrogenated depth of the first material relative to the secondmaterial.
 2. The method of claim 1, wherein the first process gasconsists essentially of hydrogen and optionally a noble gas.
 3. Themethod of claim 1, wherein the first process gas consists of hydrogen.4. The method of claim 1, wherein the first process gas consists of H₂and Ar.
 5. The method of claim 1, wherein the second process gas furtherincludes oxygen.
 6. The method of claim 1, wherein the first chemicalmixture contains hydrogen ions, and wherein the second chemical mixturecontains substantially charge-neutral species.
 7. The method of claim 1,wherein the second material is selected from the group consisting ofSiO₂ and organic materials.
 8. The method of claim 1, wherein the firstmaterial includes raised features on the substrate, the second materialforms sidewall spacers on vertical portions of the raised features onthe first and second sides of the first material, and wherein theexposing to the second plasma-excited processing gas removes the raisedfeatures of the first material but not the sidewall spacers.
 9. Themethod of claim 1, wherein the plasma excitation of the first processgas or the second process gas includes generating plasma using acapacitively coupled plasma source containing an upper plate electrode,and a lower plate electrode supporting the substrate.
 10. The method ofclaim 1, wherein the plasma excitation of the first process gas or thesecond process gas includes generating plasma using an inductivelycoupled plasma source containing an inductive element, and a lower plateelectrode supporting the substrate.
 11. The method of claim 1, whereinthe plasma excitation of the first process gas or the second process gasincludes generating plasma using a remote plasma source that creates ahigh radical to ion flux ratio.
 12. The method of claim 1, furthercomprising: repeating the steps of forming the first chemical mixture,exposing the first material to the first chemical mixture, forming thesecond chemical mixture, and exposing the first material to the secondchemical mixture to incrementally remove additional portions of thefirst material located between the second material on the first andsecond sides.
 13. The method of claim 1, wherein during the exposing ofthe first material on the substrate to the first chemical mixture, thesilicon nitride material becomes hydrogenated at a surface region and asub-surface region of the silicon nitride material; and wherein duringthe exposing of the first material on the substrate to the secondplasma-excited process gas, an etch rate of the silicon nitride materialinitially is increasing during etching of the surface region and a firstportion of the sub-surface region, and thereafter the etch rate of thesilicon nitride material is decreasing during etching of a secondportion of the sub-surface region.
 14. The method of claim 1, whereinthe first material consists essentially of SixNy, where x and y areintegers; and wherein during the exposing the first material to thefirst chemical mixture the first and second materials are not etched,and the first material is hydrogenated in surface and sub-surfaceregions of the first material, and wherein after the exposing the firstmaterial to the first chemical mixture the sub-surface regions include afirst portion having a hydrogen concentration greater than that at thesurface and a second portion having a hydrogen concentration lower thanthat at the surface.
 15. The method of claim 14, wherein the exposingthe first material to the first chemical mixture is performed at apressure lower than a pressure of the exposing the first material to thesecond plasma-excited process gas.
 16. The method of claim 1, whereinthe exposing the first material to the first chemical mixture isperformed at a pressure lower than a pressure of the exposing the firstmaterial to the second plasma-excited process gas.
 17. The method ofclaim 1, wherein the plasma processing apparatus includes a first powersource which applies power to the plasma processing apparatus at a firstfrequency, and a second power source which applies power to the plasmaprocessing apparatus at a second frequency different from the firstfrequency, and wherein during the forming of the first chemical mixtureand exposing the first material to the first chemical mixture, thesecond power source is off.
 18. The method of claim 17, wherein thefirst process gas consists essentially of hydrogen and optionally anoble gas.
 19. The method of claim 1, wherein the substrate furtherincludes a third material different from the first and second materials,wherein during the exposing of the first material to the first chemicalmixture and the exposing the first material to the second plasma-excitedprocess gas, the second and third materials are exposed, and wherein theexposing the first material to the second plasma-excited process gasselectively etches the first material with respect both the secondmaterial and the third material.
 20. The method of claim 19, wherein thethird material includes an organic material.